The present invention relates to a control system for a continuously variable belt-drive automatic transmission for a motor vehicle, and more particularly to a system for controlling the transmission ratio of the transmission.
Referring to FIG. 6, a continuously variable belt-drive transmission has an endless belt 16 running over a drive pulley 14 and a driven pulley 15. Each pulley comprises a hydraulically shiftable conical disc which is axially shifted by a fluid operated servo device so as to vary the running diameter of the belt on the pulleys in dependency on driving conditions. The system is provided with a line pressure control valve and a transmission ratio control valve. Each valve has a spool to control the oil supplied to the servo devices.
The transmission ratio control valve operates to determine the transmission ratio in accordance with the opening degree of a throttle valve of an engine and the speed of the engine. The line pressure control valve is adapted to control the line pressure in accordance with the transmission ratio and the engine speed. The output of an engine is transmitted to the drive pulley through a clutch. The line pressure is controlled to prevent the belt from slipping on the pulleys in order to transmit the output of the engine.
At the start of the vehicle, the transmission ratio is set at a maximum value. When the vehicle speed and engine speed reach set values under a driving condition, the transmission ratio starts to change (upshift). The transmission ratio is controlled to a desired transmission ratio.
However, there is a problem that the transmission ratio can not be exactly controlled becuase of reasons explained hereinafter.
When the torque of the engine increases, the belt in the slack side 16a is compressed, so that the belt is outwardly expanded as shown by a dotted line as shown in FIG. 6. However, since the length of the belt is constant, the belt acts to push the movable conical disc of the drive pulley 14 to reduce the running diameter of the pulley as shown by the dotted line. Thus, the transmission ratio is increased (downshift). As a result, it is difficult to control the transmission ratio to a desired ratio.